Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones and others. One of the goals of the semiconductor industry is to improve the performance and reduce the cost of use and acquisition of high power devices used in power transmission applications such as cellular base-station transmitters and cable-TV transmitters.
Reducing the cost and increasing the performance of power amplifier based transmitter devices can be done in a number of ways. One way to reduce cost is to increase the amount of integration present on integrated circuits. Increasing integration reduces the number of components required for purchase, reduces the amount of board space required for a particular design, and reduces the amount of labor required to test and calibrate a particular amplifier design, if necessary. Another way to reduce the cost of a power transmitter product is to incorporate features that reduce the difficulty of product design and enhance the reliability of the design.
To give one of many examples, in the field of transmitter circuits one of the most challenging aspects of designing a transmitter is optimizing the amplifier to provide acceptable gain, output match, and stability. This optimization is typically performed by adjusting external matching components. In some cases, hand tuning is required in order for these devices to have optimal performance. Hand tuning and adjustment, however, add cost to the system, and can pose support and maintenance problems if the transmitter loses calibration and adjustment in the field.
One technique that can increase the reliability and ease of use of matching networks is to create matching networks which are comprised of on-chip bond wires which reside within the package. If a matching network is included inside the integrated circuit package, performance degradation due to output matching network due to part-to-part component mismatch can be avoided, potentially yielding better signal balance and less spurious emissions.
Another challenge is providing unconditionally stable devices. An unconditionally stable device, in terms of s-parameters, provides the benefit that the device will be stable under any source or load impedance. Unconditionally stable devices are easier to design in a transmitter system and are more reliable.
One difficulty with providing an unconditionally stable device is dealing with the effect of mutual inductance between bond wires. For example, mutual inductance between input and output bond wires, or mutual inductance between the input and on-chip matching network bond wires, can provide an unwanted feedback path that destabilizes the amplifier. The effect of mutual inductance feedback becomes more pronounced at high gains, however, and can render the design of a high gain amplifier more challenging.
A number of available techniques can be used to create an unconditionally stable amplifier. One method is to include a passive loss within the amplifier. While adding a passive loss can make an amplifier unconditionally stable, the passive loss will lower the power efficiency and lower the maximum achievable gain of the amplifier. Lower amplifier gains add cost to a system because more stages of amplification, hence more components, are required for a particular gain. As more amplifier stages are added, maintaining performance, such as high linearity, becomes more challenging.